Single-Ended Triode Design
The SET amplifier is the simplest and most direct path from signal to sound. One tube, one transformer, zero feedback. This guide walks through the design of a complete single-ended triode amplifier with interactive calculators.
Why Single-Ended Triode?
The oldest topology, rediscovered for its sonic purity
A single-ended amplifier uses one active device to handle the entire audio waveform. Unlike push-pull designs that split the signal into halves, the SET output tube conducts continuously through the entire cycle. The output transformer carries both the DC plate current and the AC signal, requiring a gapped core.
SET amplifiers are prized for their even-order harmonic distortion profile, which the ear perceives as warmth and richness. The absence of crossover distortion, combined with the triode's inherently linear transfer characteristic, yields a sound often described as immediate, holographic, and emotionally engaging.
While the West moved to push-pull pentodes for power and efficiency, Japanese audiophiles preserved the art of SET amplification through the 1960s–80s. Enthusiasts like Kondo-san (Audio Note), Shindo, and Kegon championed directly-heated triodes paired with high-efficiency speakers, building a philosophy that values quality of watts over quantity.
Complete SET Amplifier
Input driver, coupling, DHT output stage, output transformer and power supply
Output Stage Calculator
Select a DHT output tube and adjust operating conditions
The optimal primary Z is typically 2× to 3× the tube's plate resistance (rp). Higher Z yields more power but increases distortion. The classic 300B uses 3\u20133.5k\u03A9 primary.
SE transformers must have an air gap to prevent DC saturation from the standing plate current. This reduces inductance, so the core must be larger than an equivalent push-pull transformer.
The output transformer is the most critical component in a SET amplifier. Premium iron from Tamura, Tango, Hashimoto, Lundahl, or Monolith Magnetics will define the amplifier's bandwidth and character.
Driving the Output Tube
Voltage amplification to swing the output grid
The simplest approach. A coupling capacitor (typically 0.22\u20131\u00B5F) blocks DC while passing the AC signal. The grid leak resistor (100\u2013470k\u03A9) provides a DC return path for the output tube's grid. Economical and effective.
A step-up interstage transformer provides voltage gain and impedance matching. This allows a low-impedance driver to deliver large voltage swings to the output grid with excellent linearity. Premium approach used in Kondo, Shindo, and other high-end designs. Typically 1:2 or 1:3 ratio.
Eliminating the coupling capacitor entirely by level-shifting the DC. Requires careful design (e.g., SRPP or CCS plate load) to set the correct DC operating point for the output grid. Purist approach with theoretical bandwidth advantages but added complexity.
Power Supply Design
The foundation of any SET amplifier
The classic topology: input capacitor (47–100µF), choke (5–10H at rated DC), output capacitor (47–100µF). The choke provides excellent ripple rejection and soft current limiting. A minimum of 60dB ripple rejection is recommended for SET, as the single-ended topology has no power supply rejection.
Directly-heated triodes use the filament as the cathode. AC heater supply will introduce 50/60Hz hum directly into the signal. DC heater supply is mandatory for DHT output tubes.
Use a dedicated low-voltage winding, bridge rectifier, and RC/regulator filter. A 10,000µF+ capacitor bank is typical. Hum pot (100Ω) for fine adjustment of the virtual center-tap.
Classic SET Amplifiers
Proven designs with component values and operating points
Full, warm, liquid midrange. The gold standard for SET.
Sweet, intimate, superb for vocals and small ensembles.
Magical purity and transparency. The purist’s choice.
Speakers for SET
Low power demands high efficiency
A 300B SET producing 7W into a typical 87dB/W/m speaker yields only about 95dB peak SPL at 1 meter. For realistic listening levels in a medium room, speakers with a sensitivity of 95dB+ are essential. With a 2W 45 SET, look for 98\u2013100dB+.
Horn-loaded speakers are the traditional partner for SET amplifiers. The horn provides acoustic amplification (95–110dB/W/m), transforming a few watts into room-filling sound. Klipsch, Altec Lansing, JBL (vintage), and modern designs from Avantgarde, Cessaro, and Living Voice.
Single full-range drivers (Lowther, Fostex, Voxativ) in back-loaded horns are a favorite pairing. No crossover means zero phase shift and perfect coherence. Sensitivity typically 93\u201398dB. The purist's match for a 45 or 2A3 SET.
Low-impedance speakers (4Ω nominal with 3Ω dips), speakers with complex crossovers and low sensitivity (<90dB), and electrostatic panels are generally poor matches for SET amplifiers. The SET needs a cooperative, efficient load.
Key SET Equations
Essential formulas for single-ended triode design
Maximum undistorted output. Vswing limited by cutoff and saturation.
Must stay below rated maximum at all times. In SE Class A, idle Pd is near maximum.
Unbypassed cathode resistor reduces gain: divide by (1 + gmRk).
Sets the operating point. Power rating: P = Ia² × Rk × 2 (safety margin).
For 3.5kΩ primary into 8Ω: N = √(3500/8) ≈ 21:1.
S in dB/W/m, P in watts. +3dB per doubling of power.
Test Your Knowledge
Validate your understanding of single-ended triode amplifier design.
What type of harmonic distortion characterizes SET amplifiers?